The present invention relates to knitting machines and more particularly to knitting machines where the needle selection is performed electromechanically with the aid of data supplied by a numerical programme, for example, or by a magnetic tape or even by a punched tape.
In these knitting machines, each needle is generally controlled by a jack for selectively moving the needle to the working position. The selection mechanisms controlled by an electromagnet have the task of placing the jack as required in the active position, from which position a cam moves the jack in the needle direction to place the needle directly or indirectly into its operative position. Indeed, in several known mechanisms, the jack moves the needle until a needle butt engages a cam which then carries it to the operative position.
Some of the known devices subject the selector member to a recovery spring which constantly tends to bias this member to the rest position. This arrangement requires an electromagnet sufficiently strong to overcome the bias of the recovery spring.
Other devices do not use a recovery spring to return the selector member to the rest position but two electromagnets and a permanent magnet attached to the selector member. Thanks to this arrangement, the selector member moves from one electromagnet to the other when their polarity is inverted. Here the use of a spring is avoided by adding a second electromagnet.
A further known device uses two counteracting springs associated with each needle. One of these springs tends to hold the jack constantly in the selection position. At each feed, the second of said springs engages a fixed cam which cocks it with centrifugal movement to move it to the proximity of a fixed selection electromagnet. If the needle should not be selected, this electromagnet repels said second spring which then engages a ramp of a second cam which moves it in a centripetal direction. During this movement, this spring presses the jack against the action of said first spring and thus separates it from the upthrow cam. Otherwise, the electromagnet holds this second spring until it engages with a separation ramp of the second cam, thereby preventing it from operating against the first spring, so that the jack is raised by the upthrow cam.
With this device, the electromagnet works only on contact with the spring, which is an advantage with respect to the devices where the electromagnet must attract the spring itself.
However, this construction has many other disadvantages. In the first place, it only allows for selection at a single level per feed. Each needle has to be provided with two counteracting springs. Consequently, the number of springs is equal to twice the number of needles. The cocking spring works under disadvantageous conditions, since it has to overcome the bias of the levelling spring.
It is shown that in this construction, the means allowing a reduction of the power of the electromagnet afford disadvantages which are substantially as troublesome as those they allow to be overcome, so that the solution proposed does not provide real technical progress.
Also, certain knitting machines use selection at several levels by disposing several stacked selectors per feed, in order to increase the time available for performing selection, giving the possibility of accelerating the speed of relative movement of the selectors and of the knitting head carrying the needles and, consequently, of increasing production. Therefore, it is important that the selection devices used should be neither too large nor too expensive, whilst still providing complete operational reliability. The space occupied and the price of the electromagnets is not negligible if one considers that a machine may have, for example, 48 feeds of 10 selectors each, making a total of 480 selectors. If each selector uses two electromagnets as in one of the aforementioned solutions, 960 would be needed.